The field of the invention is programmable controllers, and particularly, microprocessor-based programmable controllers such as those disclosed in U.S. Pat. No. 4,165,534 entitled "Digital Control System with Boolean Processor," and co-pending U.S. patent application Ser. No. 43,897 entitled "Mini-Programmable Controller," and Ser. No. 026,012 entitled "Microprogrammed Programmable Controller."
Since their inception, programmable controllers have been clearly distinguishable from other programmable processors by their instruction set which is tailored to meet the specific needs of the control engineer. This instruction set provides a language which is easily understood and used by those who have heretofore designed systems of relays, switches and motors using ladder diagrams. Not only are programmable controller processors, such as those described above, designed to execute this special purpose instruction set, but program loaders such as those disclosed in U.S. Pat. Nos. 3,798,612; 3,813,649; and 4,070,702 have been developed to facilitate the loading and editing of control programs using this instruction set.
Microprocessor-based programmable controllers employ processor circuits which are available commercially in integrated circuit form. Such well known microprocessors as the Intel 8008 and 8080, the Zilog Z-80, the Motorola 6800 and the Advanced Micro Devices Am2910 are examples of a few of such circuits. These microprocessors each have their own particular instruction set which is characterized by its general applicability to a wide variety of data processing uses. When such a microprocessor is used in a programmable controller, therefore, the control program which is comprised of programmable controller-type instructions must be converted, or translated, into the machine language of the particular microprocessor which is used.
The use of microprocessors and knowledge of their programming languages has become widespread in recent years. As a result, larger numbers of programmable controller users have developed the capability of employing microprocessors to solve special control problems which may be outside the scope of not only programmable controllers, but other commercially available control systems. On the other hand, users who do not have such capability make continuous demands upon the manufacturers of programmable controllers to expand their instruction sets to enable them to more readily solve their special control problems. Where the control problem is unique to only a few users, the expansion of the instruction set usually cannot be justified in prior programmable controllers and the user is left to his own resources.